Dear Jaguar owners,
I have a 1994 XJ12 with a 6.0 ltr engine. The car has been sat for a few years, although I used to run it in the garage for a few minutes every few months. Lately I decided to start using it for the occassional sunday ride. I did an oil and coolant change. However, while driving I started noticing that once the coolant temperature gauge reaches the "N" the cooling fans would kick in and stay on for the rest of the ride, and also till about 5 to 10 minutes after the engine is turned off. The Temperature gauge always reaches the "N" and stays there, whether it is a hot or cold day, and there is no overheating smell or boiling of coolant. I also checked the upper radiator hoses and they are both hot after a good run, so coolant must be flowing throught them.
I did some research and I found that it could be a faulty tmperature sensor so I had this replaced, but still, once the fans kick in, they remain on.
I do not think that it is the relay as otherwise the fans would not turn on at all, or would never turn off. In my case after a few minutes of the engine switch of, the fans switch off too, so the relay must be functioning.
I did an OBDII scan and no faults came up and also there are no warning lights on the dash. There are no coolant leaks and the reservoir is filled to the appropriate level.
I am suspecting a thermostat stuck at half open, which is allowing just enough coolant to pass to relieve the engine from excessive heat, but not enough for the radiator to cool properly before re-enterring the engine. What is not worrying is that the temperature gauge does not go above the "N" but obviosuly with a fan constantly running I am risking a fan burnout and also a dead battery.
However, before going to the trouble of replacing the thermostats, as there are two thermostats on the v12, and not much space to work, is there someone who has an idea if there could also be something else wrong? I would like to clarify that the fans do not kick in immediately on switching the negine on - only after the engine has properly warmed up and the temperature reaches the normal.
Thanks in advance

 

G’day Blaise,

That all sounds normal to me.

The fans come on at 86degC and will run for sometime after the engine is shut down until it cools down.

The temp gauge has been tricked to always read close to N and very rarely moves once the engine is warmed up.

Cheers and enjoy the ride,
Jeff.

 

Dear Jeff,
Thanks for your reply. The fans staying on for a while after the engine is turned off I understand. What is bothering me is that they are running even as I drive. Once the temperature reaches the N, the fans will start running and if I have an hour on the road, they will run for the full hour without cutting out.
Now also, if the gauge is as you say tricked to always read close to N then it is not properly a good indicator if the engine is overheating or not which all the more makes me think that I should have the thermostats replaced to make sure they are working properly.

 

G’day Blaise,

All sounds correct. The thermostats are usually 88degC and the fans come on at 86 degC so they should always run. At 100degC they go to high speed.

Cheers,
Jeff.

 

Hi Jeff,

Hmmm I would have thought that the electric fans would only work under certain conditions, such as for example when stuck in traffic and there is not enough air flowing in to cool the radiator and the clutch mounted fan is not enough, or for instance when driving during very hot climate.
Anyway, I have decided to go down the route of replacing the thermostats. It's a 30+ year old car and for all I know the thermostats are still the factory originals, so changing them shouldn't do any harm.
But thanks for your input. At least I can rest my mind that I do not have anything serious

 

Before firing the parts cannon, you might as well monitor coolant temps via OBDII.

In my case, I noticed that the old thermostats would allow flow down into the 60s °Celsius on long highway downhills.

 

G'day Blaise,

Here is a link to a lot of Jag literature, you will find all the answers within.

https://www.jaguarforums.com/forum/x...48/#post263943

Cheers,
Jeff.

 

Thermostats being in good temperature tolerance is critical on the V - 12 even if ordering the correct ones ( 2 )

Someone has made a good PDF on the V - 12 thermostats

To know the radiator cooling fans are operating correctly have the ignition key in run but engine not started , remove the 3 wire coolant temp switch connector on the radiator's rear car's left lower

Jumper one white wire to the black and you will get low fans mode

Remove jumper and jumper the other white wire to the black and you will get high fans mode

Low current portion of circuit so paper clip fine

This does not test the 3 wire temp switch itself so water bath test , the X308 black color fans control relay module does not work for the X300 which is white in color